Yahoo can be set up manually (IMAP) in the stock client and seems to work better with 2.2 than it did with 2.1. The below works perfectly for me. I found this on another forum, I don't recall which one.
Incoming settings:
IMAP Server: android.imap.mail.yahoo.com
Port: 993
Security type: SSL
IMAP Path prefix: [leave this blank]
Outgoing settings:
SMTP Server: android.smtp.mail.yahoo.com
Port: 465
Security type: SSL
Make sure the box is ticked for 'Require sign-in'